Discussion:
The results of our study suggest that TMPG correlates poorly with the
severity of MS in patients with DMS. Moreover, DMSI may be a viable
alternative for the evaluation of DMS. The potential inadequacy of mean
TMPG reported here agrees with previous
guidelines.2,17 However, our results note a
significant discordance between TMPG and MVA in DMS patients.
Differences in MV morphology and net atrioventricular compliance between
DMS and RMS may, in part, explain this observation. The flat plate
geometry caused by MAC, compared to the dome-shape of RMS, increases the
contraction coefficient.14 An increased contraction
coefficient causes a lower pressure drop across the valve, accounting
for the low transvalvular gradient.10,11,14 Also, risk
factors for an increased net atrioventricular compliance are much more
common in the patients with DMS than RMS.11,20–23 An
increased net atrioventricular compliance accelerates the rate of
transvalvular pressure decay, hence lowering the mean
TMPG.20,24 A recent study suggested the presence of
diastolic dysfunction as a potential cause of low TMPG in patients with
very severe RMS.25 The above factors may be
responsible for lower mean TMPG seen in DMS patients in our study. The
use of mean TMPG as an index for MS severity carries a 1B
recommendation, but it may not accurately reflect MS severity in the
setting of DMS.15–17.
The patients in our RMS and DMS groups were similar to each other
regarding some of the hemodynamic (i.e., pulmonary artery systolic
pressure, SV index) and structural (i.e., left atrial volume index)
parameters. Despite this similarity between groups, DMS was associated
with a larger MVA and lower mean TMPG compared to RMS. Consistent with
our findings, Pressman et al. recently reported in a retrospective
cohort study that DMS is associated with a larger MVA relative to mean
TMPG when compared to RMS.26
The DMSI proposed here denotes an increased velocity across the stenotic
MV relative of the LVOT velocity. Similar to the continuity equation,
the DMSI relies on the rule of conservation of mass. The strong
correlation between MVACEQ and the DMSI relies on the
fact that the DMSI is a large component of the MVACEQformula. Despite these similarities, this DMSI has several theoretical
advantages. It avoids the variability of the LVOT cross-sectional area
and reduces error risk driven by LVOT diameter measurement and
assumptions of LVOT shape. Thus, the index would be most useful in cases
with suboptimal visualization of the LVOT.
The DMSI is relatively flow independent in the absence of significant
valvular regurgitation. Therefore it would not be affected by the heart
rate or flow state. The concept of the velocity ratio was previously
described in the assessment of prosthetic aortic and mitral valve
stenoses.27–29 The former utilized the ratio of LVOT
VTI to prosthetic valve VTI with < 0.30 denoting severe
prosthetic aortic valve stenosis, and the latter utilized ratio of
mitral prosthesis VTI to LVOT VTI denoting > 2.5 as a
significant valve dysfunction.29 Furthermore, the
dimensionless index for the native aortic valve is calculated as a ratio
of LVOT VTI to aortic valve VTI. Moreover, an index of < 0.25
is consistent with severe aortic valve stenosis.
A recent study by Cho et al. including RMS patients with very severe MS
(MVA ≤ 1 cm2) undergoing MV replacement showed that patients with low
mean TMPG (≤ 10 mmHg) were more likely to be older, and females, and
have diabetes or AF compared to those with high mean TMPG (≥10
mmHg).25 The authors noted that this might be related
to higher atrial compliance and lower atrial pressure seen in RMS
patients. Interestingly, LV SV was higher in the low mean TMPG group in
the study mentioned above. The frequency of low-flow state (SV index
<35 ml/m2) was similar in our DMS and RMS
groups. Therefore, between-group differences (mean TMPG and
MVACEQ) observed in our study is unlikely to originate
from differences in flow-states. We did not perform further subgroup
analyses for patients with low-flow states because of the small sample
size. Cho et al. reported an association between AF and low mean TMPG in
patients with RMS.25 Absence of such a relationship in
our study might be due to the small number of patients with AF in our
DMS group (14%).
Our study shows that mean TMPG does not correlate well with
MVACEQ and that a practical tool such as DMSI proposed
in this study may serve as a simile echocardiographic parameter in the
identification of patients with significant DMS.
Our survival analysis indicated that patients with DMS have a poor
prognosis, and more than 50% of the patients with DMS died over an
average of 13 months. The small sample size was a limitation for the
detailed evaluation of predictors of mortality among patients with DMS.
According to the cox-regression model, the left atrial volume index and
ESRD were the only two independent predictors of mortality among DMS
patients. A non-significant trend towards increased mortality risk was
observed with MVACEQ ≤ 1.0 cm2 and
DMSI ≤ 0.35. Other echocardiographic parameters such as
MVACEQ ≤ 1.5 cm2, mean TMPG
> 7 mmHg, or SV index ≤ 0.35 did not predict mortality
risk.
Medical management of DMS includes heart rate control, and diuretic
therapy that may cause provide some symptomatic
relief.2 Surgical treatment for severe DMS is
significantly challenging due to older age of these patients with
multiple comorbidities.2 Emerging transcatheter
approach for mitral valve replacement is feasible, but further
advancement is needed to improve outcomes.30